It is commonplace in the field of borehole formation, for subsequent use in the extraction of hydrocarbons, to use a steerable drilling system to allow control over the path of the borehole. A number of steerable drilling systems are known. For example systems are known in which a bias unit is located close to the drill bit, the bias unit being operable to apply a laterally directed load to the drill bit urging it away from the axis of the borehole in a desired direction. Another form of steerable drilling system includes a bent housing upon which the drill bit is mounted, steering of the system being achieved by controlling the orientation of the bent housing to ensure that the drill bit is pointed in a desired direction. In some arrangements the bent housing is adjustable to control the direction and angle of the drill bit. In other arrangements, the bent housing is of fixed inclination and adjustment of the direction in which the drill bit is pointed is achieved by controlling the angular position of the bent housing.